The invention relates to apparatus for manufacturing a broadband cholesteric polarizer, in which a liquid-crystalline, cholesterically ordered layer comprising reactive chiral monomers and reactive nematogenic monomers of different reactivity is polymerized by exposure to radiation.
Broadband cholesteric polarizers and methods of manufacturing same are polarizers it is possible to convert unpolarized light to circularly polarized light in a substantially loss-free manner. Polarizers of this type comprise a thin layer of a cholesterically (i.e. chirally nematically) ordered material. This material contains chiral, liquid-crystalline molecules having such a structure that they order themselves more or less spontaneously into a spiral-shaped or helical structure. The pitch of this helix can be increased by adding a quantity of a non-chiral, liquid-crystalline (i.e. nematogenic) material to the chiral, liquid-crystalline material. The exact pitch is governed by the ratio between the quantities of chiral and non-chiral liquid-crystalline molecules as well as by their chemical structure.
If this material is provided in the form of a thin layer on a substrate or between two substrates, the helical structure assumes such an orientation that the axis of the helix extends transversely to the layer. Such a layer is capable of reflecting a narrow band of light whose wavelength corresponds to the product of the pitch and the refractive index of the material and whose direction of polarization corresponds to the handedness of the helical structure. By virtue of this property, a cholesteric layer can very suitably be used in an optical polarizer. It is noted that the expression xe2x80x9cthe refractive indexxe2x80x9d of a material is to be understood to mean in this context the geometric mean (ne+no)/2 of the ordinary refractive index no and the extraordinary refractive index ne of this material.
Broadband cholesteric polarizers are distinguished from the customary cholesteric polarizers by the presence of a relatively broad reflection band. The bandwidth of the customary cholesteric polarizers is only approximately 40-50 nm. In the case of broadband polarizers, bandwidths of 100 nm, 150 nm, 200 nm and even more than 400 nm have been achieved. It is noted that the band position of a cholesteric filter is defined as the center of the wavelength range in which the reflection takes place. A width of a band is defined as the difference in wavelength between the long-wave and the short-wave edge positions of the band. The wavelength of an edge position is defined as the wavelength at which the intensity amounts to 50% of the maximum intensity.
EP 606940 describes an elegant method of manufacturing a broadband cholesteric polarizer. Use is made of a mixture comprising reactive chiral monomers and reactive nematogenic monomers, which exhibit a different reactivity. For the reactive monomers use can be made of compounds containing a reactive group on the basis of acrylates, epoxy compounds, vinylethers and thiolene systems, as described, inter alia, in U.S. Pat. No. 5,188,760. Monomers containing different reactive groups generally exhibit a different reactivity. A difference in reactivity also occurs if one type of monomers contains one reactive group and the other type of monomers contains two (identical) reactive groups.
A layer of this mixture is polymerized by means of (actinic) radiation, in particular UV radiation. In this process, the conditions are selected in such a manner that during the polymerization operation a radiation profile of varied intensity is formed in the layer. As a result, diffusion processes take place in the cholesteric layer during polymerization. This leads to a variation in the composition of the helical structure, so that the pitch, viewed across the thickness of the layer, varies within certain limits. As a result, this cholesteric polarizer exhibits a relatively broad reflection band.
It has been found that the method described in EP-A 696940 can be improved. The Applicant has experimentally established that small fluctuations, for example in the radiation gradient or in the UV intensity, can strongly influence the diffusion processes of the reactive monomers. This may lead to relatively large differences in the bandwidth of the cholesteric polarizers manufactured by means of said known method. Therefore said known method should be improved, in particular, with respect to the reproducible manufacture of polarizers having a correct position of one of the two edges of the reflection band. This applies, for example, to polarizers as described in EP-A 95203209.2(U.S. Pat. No. 5,825,444).
It is an object of the invention to improve the known method. The invention more particularly aims at providing a method of manufacturing broadband cholesteric polarizers of which the position of one of the two edges of the reflection band can be adjusted in a very reproducible manner. The method in accordance with the invention should enable these polarizers to be mass-produced.
These and other objects of the invention are achieved by substantially increasing the intensity of the radiation when the band reaches a desired edge position.
The invention is based on the experimentally gained insight that the intensity of the radiation used during polymerization plays an important part in the manufacture of broadband polarizers. It has been demonstrated that the eventually achieved bandwidth is governed to a substantial degree by the radiation intensity used. If use is made of a relatively high UV intensity (typically 0.5 mW/cm2 or higher), the eventually achieved bandwidth is found to be relatively small, and it differs hardly from that of the unpolymerized mixture. If a relatively low radiation intensity (typically 0.05 mW/cm2 or lower) is used, a much broader reflection band is obtained. Under these conditions, first, a colored, narrow reflection band is formed, which subsequently broadens into an uncolored, broadband reflection band. A substantial increase of the intensity causes the bandwidth obtained at that instant to be frozen, as it were. It has been demonstrated that the increase in intensity should preferably be a factor of 10 or more to bring about the frozen state. Preferably, this factor is 20 or more. Under these conditions, the cholesterically ordered layer instantly becomes completely polymerized.
A preferred embodiment of the method in accordance with the invention is characterized in that the attainment of the desired edge position of the band is determined by means of a monochromatic photosensor, the wavelength used by the sensor corresponding to the wavelength of the desired edge position of the band. Such a photosensor comprises a photodetector as well as a monochromatic light source. A laser can very advantageously be used as the monochromatic light source in the sensor.
The sensor can be used in reflection. Said sensor is constructed in such a manner that the monochromatic light, which emanates from the light source and which is used in the measuring operation, is directed to the layer to be polymerized. As long as the wavelength of the edge position of the reflection band is not equal to that of the monochromatic light, this light will pass through the layer to be polymerized (transmission). As soon as the bandwidth assumes such a value that the two wavelengths coincide, reflection of the monochromatic light occurs. A proper positioning of the layer, the light source and the detector causes this light to be reflected toward the detector. At this moment, the intensity of the polymerization radiation should be increased. To this end, a second polymerization lamp having a higher radiation intensity is activated or, preferably, a filter situated in front of the polymerization lamp is removed. Instead of a (mechanically) movable filter, use can also advantageously be made of a filter whose transmission is adjustable.
Another preferred embodiment of the method in accordance with the invention is characterized in that the desired edge position of the band is determined by means of a transmission measurement. For this purpose, the sensor is constructed so that the light source and the detector are situated on either side of the layer to be polymerized. In this case, the sensor is activated as soon as the detector detects a substantial reduction in intensity of the light emanating from the light source. Such an arrangement has the advantage that the exact alignment of the layer to be polymerized does not affect the measuring results of the sensor.
Another suitable embodiment of the method in accordance with the invention is characterized in that the liquid crystalline, cholesterically ordered layer is passed through an illumination tunnel which is provided with a number of compartments which comprise a radiation source as well as a light sensor by means of which the intensity of the radiation incident on the layer can be changed. This embodiment of the method in accordance with the invention enables broadband, cholesteric polarizers to be manufactured in a continuous process. This has a favorable effect on the cost price per unit area.
The invention also relates to a device for manufacturing a broadband cholesteric polarizer, which is characterized by a radiation compartment comprising
a) means for positioning the polarizer to be manufactured,
b) a radiation source for the irradiation of the polarizer to be manufactured and
c) a monochromatic photosensor comprising a photodetector as well as a monochromatic light source.
This device in accordance with the invention enables broadband polarizers to be manufactured in batch processes. Preferably, the photodetector and the monochromatic light source are positioned in the radiation compartment in such a manner that the sensor measures in the transmission. To this end, the polarizer to be manufactured is situated between the detector and the light source during operation of the device.
Another device in accordance with the invention, which is used to manufacture a broadband cholesteric polarizer, is characterized in that the device comprises a number of radiation compartments, which are provided with
a) means for passing a substrate through the radiation compartments,
b) a radiation source for irradiating the substrate to be passed through said compartments, and
c) a monochromatic photosensor which comprises a photodetector as well as a monochromatic light source.
This device in accordance with the invention enables broadband polarizers to be manufactured in a continuous process. Preferably, the photodetector and the monochromatic light source are positioned in the radiation compartments in such a manner that the sensors measure in transmission. For this purpose, thexe2x80x94preferably elongatedxe2x80x94substrate to be passed through the radiation compartments is situated between the detector and the light source during operation of the device.
To increase the radiation intensity, use can be made, for example, of a second radiation source having a higher intensity. This source should be driven by the photosensor. A cheaper preferred embodiment of both devices in accordance with the invention is however characterized in that the compartments comprise an optical filter which can be displaced by driving the sensor. Such a filter preferably passes 10% or less of the radiation produced by the lamp. It is noted that an adjustable-transmission filter can be used instead of a displaceable filter.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.